Composite material for heat sinks for semiconductor devices and method for producing the same

ABSTRACT

A high-pressure vessel is allowed to be in an initial state, and a first chamber is disposed downward. Copper or copper alloy is placed in the first chamber, and SiC is set in a second chamber. The high-pressure vessel is tightly sealed, and then the inside of the high-pressure vessel is subjected to vacuum suction through a suction pipe. An electric power is applied to a heater to heat and melt the copper or copper alloy in the first chamber. At a stage at which the molten copper in the first chamber arrives at a predetermined temperature, the high-pressure vessel is inverted by 180 degrees to give a state in which SiC is immersed in the molten copper. An impregnating gas is introduced into the high-pressure vessel through a gas inlet pipe to apply a pressure to the inside of the high-pressure vessel. Thus, SiC is impregnated with the molten copper. The high-pressure vessel is inverted by 180 degrees, and then the impregnating gas in the high-pressure vessel is discharged through a gas outlet pipe, simultaneously with which a cooling gas is introduced into the high-pressure vessel through the gas inlet pipe to cool the high-pressure vessel.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a composite material and a method forproducing the same, the composite material being used for heat sinks forsemiconductor devices, for constructing a heat sink for a semiconductordevice for efficiently releasing heat generated from the semiconductordevice.

2. Description of the Related Art

In general, heat is a dangerous enemy of semiconductor devices.Therefore, it is necessary that the internal temperature of thesemiconductor device does not exceed a maximum allowable temperature forretaining the joining or connecting structure. Semiconductor devicessuch as power transistors and semiconductor rectifying elements consumea large amount of electric power per unit of operation area. Therefore,it is impossible to release a sufficient amount of the generated heatonly by relying on an amount of heat released through a case (package)and lead wires of the semiconductor device. In such a circumstance,there is a fear that the internal temperature of the device is raised,and thermal destruction would occur.

This phenomenon also occurs in the same manner in semiconductor deviceswhich carry a CPU. The amount of heat generation during operation isincreased in proportion to the improvement in clock frequency. As aresult, it is an important factor to make a thermal design inconsideration of heat release.

In the thermal design in consideration of avoidance of the thermaldestruction or the like, element designs and mounting designs are madetaking account of a heat sink having a large heat release area which issecurely attached to a case (package) of the semiconductor device.

In general, those used as the material for the heat sink include metalmaterials such as copper and aluminum having good thermal conductivity.

Recently, in the semiconductor devices such as CPUs and memories, it isintended to drive the device with low electric power in order todecrease electric power consumption, while the semiconductor deviceitself tends to have a large size in proportion to highly densifiedelement integration and enlargement of element formation area. When thesemiconductor device has a large size, the stress, which is generateddue to the difference in thermal expansion between the semiconductorsubstrate (silicon substrate or GaAs substrate) and the heat sink, isincreased. As a result, there is a likelihood of delamination andmechanical destruction of the semiconductor device.

In order to avoid such inconveniences, the conceivable countermeasureincludes realization of low electric power operation of thesemiconductor device and improvement of the material for heat sinks. Atpresent, in relation to the low electric power operation of thesemiconductor device, a device, which is operated at a power sourcevoltage of a level of not more than 3.3 V, is practically used, beyondthose operated at the TTL level (5 V) having been hitherto used.

On the other hand, in relation to the constitutive material for the heatsink, it is insufficient to consider only the thermal conductivity.Besides, it is necessary to select a material having high thermalconductivity with a coefficient of thermal expansion which isapproximately coincident with those of silicon and GaAs to be used forthe semiconductor substrate.

A variety of reports have been submitted in relation to the improvementin material for the heat sink. For example, there is a case based on theuse of aluminum nitride (AlN) and a case based on the use of Cu(copper)-W (tungsten). AlN is excellent in balance between the thermalconductivity and the thermal expansion, and it especially has acoefficient of thermal expansion which is approximately coincident withthat of Si. Therefore, AlN is preferred as a material for heat sinks forthe semiconductor device based on the use of a silicon substrate as thesemiconductor substrate.

On the other hand, Cu-W is a composite material which possesses both thelow thermal expansion of W and the high heat conductivity of Cu, and iteasily processed by means of machining. Therefore, Cu-W is preferred asa constitutive material for heat sinks having complicated shapes.

There are other suggested cases including, for example, a materialobtained by containing metallic Cu in a ratio of 20 to 40% by volume ina ceramic base material comprising a major component of SiC(Conventional Example 1: see Japanese Laid-Open Patent Publication No.8-279569), and a material obtained by impregnating a powdery sinteredporous body comprising inorganic substances with Cu in an amount of 5 to30% by weight (Conventional Example 2: see Japanese Laid-Open PatentPublication No. 59-228742).

The material for heat sinks concerning Conventional Example 1 is basedon powder shaping in which a green compact comprising SiC and metallicCu is shaped to prepare a heat sink. Therefore, the coefficient ofthermal expansion and the coefficient of thermal conductivity thereofare persistently represented by theoretical values. In this case, thereis a problem that it is impossible to obtain the balance between thecoefficient of thermal expansion and the coefficient of thermalconductivity demanded for actual electronic parts or the like.

In Conventional Example 2, the ratio of Cu, with which the powderysintered porous body comprising inorganic substances is impregnated, islow. Therefore, there is a fear that a limit appears when it is intendedto increase the thermal conductivity.

SUMMARY OF THE INVENTION

The present invention has been made taking such problems intoconsideration, an object of which is to provide a composite material forheat sinks for semiconductor devices, which makes it possible to obtaincharacteristics adapted to balance the coefficient of thermal expansionand the coefficient of thermal conductivity demanded for actualelectronic parts or the like (including semiconductor devices).

Another object of the present invention is to provide a method forproducing a composite material for heat sinks for semiconductor devices,which makes it possible to easily perform a treatment for impregnating aporous sintered compact with a metal although such a treatment isgenerally considered to be difficult, making it possible to improve therate of impregnation of the metal into the porous sintered compact, andmaking it possible to improve the productivity of the heat sink whichhas characteristics adapted to balance the coefficient of thermalexpansion and the coefficient of thermal conductivity demanded foractual electronic parts or the like (including semiconductor devices).

At first, explanation will be made for the optimum characteristics asthe material for heat sinks. The required coefficient of thermalexpansion is preferably in a range of 4.0×10⁻⁶ /° C. to 9.0×10⁻⁶ /° C.as an average coefficient of thermal expansion from room temperature to200° C., because it is necessary to conform to the coefficient ofthermal expansion of the ceramic substrate such as those composed of AlNand the semiconductor substrate such as those composed Si and GaAs. Therequired coefficient of thermal conductivity is preferably not less than180 W/mK (room temperature), because it is necessary to satisfy therequirement equivalent or superior to those satisfied by the presentlyused Cu-W material.

According to the present invention, there is provided a compositematerial for heat sinks for semiconductor devices, comprising a poroussintered compact impregnated with copper or a copper alloy, the poroussintered compact being obtained by pre-calcinating a porous body havinga coefficient of thermal expansion which is lower than a coefficient ofthermal expansion of copper so that a network structure is formed;wherein the composite material has a characteristic that at least acoefficient of thermal expansion at 200° C. is lower than a coefficientof thermal expansion which is stoichiometrically obtained on the basisof a ratio between the copper or the copper alloy and the poroussintered compact.

According to the present invention, it is possible to suppress theexpansion to be at a value which is lower than the thermal expansion(theoretical value) determined by the ratio between the porous sinteredcompact and the copper or the copper alloy with which the poroussintered compact is impregnated. The coefficient of thermal expansion isapproximately coincident with those of, for example, ceramic substratesand the semiconductor substrates (silicon, GaAs). Thus, it is possibleto obtain a material for heat sinks having good thermal conductivity.

Specifically, it is possible to obtain a material for heat sinks inwhich an average coefficient of thermal expansion in a range from roomtemperature to 200° C. is 4.0×10⁻⁶ /° C. to 9.0×10⁻⁶ /° C., and acoefficient of thermal conductivity is not less than 180 W/mK (roomtemperature).

It is desirable that the porous sintered compact comprises at least oneor more compounds selected from the group consisting of SiC, AlN, Si₃N₄, B₄ C, and BeO. It is desirable that the ratio (impregnation rate) ofthe copper or the copper alloy is 20% by volume to 70% by volume. If therate of impregnation of copper is less than 20% by volume, it isimpossible to obtain the coefficient of thermal expansion of 180 W/mK(room temperature), while if the rate of impregnation exceeds 70% byvolume, then the strength of the porous sintered compact (especiallySiC) is lowered, and it is impossible to suppress the coefficient ofthermal expansion to be less than 9.0×10⁻⁶ /° C.

It is desirable that a value of an average open pore diameter of theporous sintered compact is 0.5 to 50 μm. If the value of the averageopen pore diameter is less than 0.5 μm, then it is difficult toimpregnate the open pores with the metal, and the coefficient of thermalconductivity is lowered. On the other hand, if the value of the averageopen pore diameter exceeds 50 μm, then the strength of the poroussintered compact is lowered, and it is impossible to suppress thecoefficient of thermal expansion to be low.

It is preferable that the distribution (pore distribution) in relationto the average open pore of the porous sintered compact is distributedby not less than 90% in a range of 0.5 to 50 μm. If the pores of 0.5 to50 μm are not distributed by not less than 90%, the open pores, whichare not impregnated with copper, are increased. Consequently, thecoefficient of thermal conductivity is decreased, or the strength isdecreased, and it is impossible to suppress the coefficient of thermalexpansion to be low.

It is desirable that bending strength of the porous sintered compact isnot less than 10 MPa. If the strength is less than this value, then itis impossible to suppress the coefficient of thermal expansion to below, and it is impossible to obtain the composite material having thecoefficient of thermal expansion in the predetermined range.

In general, when commercially available pure copper is used as thecopper, a good composite material having a high coefficient of thermalconductivity is obtained. However, the obtained composite material isnot excellent in wettability with respect to the porous sintered compact(especially SiC), and the open pores which are not impregnated withcopper tend to remain. Therefore, it is desirable to improve theimpregnation rate by adding, for example, Be, Al, Si, Mg, Ti, and Ni.However, if the additive is added in an amount of not less than 1%, thenthe coefficient of thermal conductivity is greatly decreased, and it isimpossible to obtain the effect which would be otherwise obtained by theaddition.

It is desirable that a reaction layer, which is formed at an interfacebetween the porous sintered compact and the copper (only copper or onecontaining copper and Be, Al, Si, Mg, Ti, Ni or the like in a range upto 1%), is not more than 5 μm. More preferably, the reaction layer isnot more than 1 μm. If the reaction layer is thicker than 5 μm, then theheat transfer between the porous sintered compact and the copper isdeteriorated, and the thermal conduction of the composite material forheat sinks for semiconductor devices is decreased.

In another aspect, the present invention provides a method for producinga composite material for heat sinks for semiconductor devices, themethod comprising an impregnating step of heating a porous sinteredcompact to serve as a base material and a metal containing at leastcopper, in a state of making no contact with each other, and makingcontact with both at a stage of arrival at a predetermined temperatureto immediately apply a high pressure so that the porous sintered compactis impregnated with the metal; and a cooling step of cooling the poroussintered compact impregnated with at least the metal.

For example, the porous sintered compact to serve as the base materialand the copper or the copper alloy used for impregnation thereof areheated while making no contact with each other. At the stage at whichboth arrive at a temperature not less than a melting point of the copperor the copper alloy, both are allowed to make contact with each other toimmediately apply the high pressure so that the porous sintered compactis impregnated with the copper or the copper alloy, followed by quickcooling.

Accordingly, the treatment for impregnating the porous sintered compactwith the copper or the copper alloy, which is generally considered to bedifficult, can be performed with ease. Moreover, it is possible toimprove the rate of impregnation of the copper or the copper alloy intothe porous sintered compact. As a result, it is possible to improve theproductivity of the heat sink which has characteristics adapted tobalance the coefficient of thermal expansion and the coefficient ofthermal conductivity demanded for actual electronic parts or the like(including semiconductor devices).

The characteristics adapted to balance the coefficient of thermalexpansion and the coefficient of thermal conductivity demanded foractual electronic parts or the like (including semiconductor devices)are represented such that the average coefficient of thermal expansionfrom room temperature to 200° C. is 4.0×10⁻⁶ /° C. to 9.0×10⁻⁶ /° C.,and the coefficient of thermal conductivity is not less than 180 W/mK(room temperature).

In a preferred embodiment, the impregnating step may comprise the stepsof placing the porous sintered compact and the metal into an identicalvessel, arranging the metal at a lower portion of the vessel, and thenallowing the vessel to be in a negative pressure state or in an ordinarypressure state therein; heating and melting the metal to convert themetal into molten metal; inverting the vessel at a stage at which themolten metal arrives at a predetermined temperature to immerse theporous sintered compact in the molten metal in the vessel; andimpregnating the porous sintered compact with the molten metal byintroducing an impregnating gas into the vessel to apply a pressure inthe vessel.

That is, the porous sintered compact and the copper or the copper alloyto be used for impregnating the porous sintered compact therewith areplaced in the vessel, and the vessel is tightly sealed to perform vacuumsuction, followed by heating while placing the copper or the copperalloy at the lower portion of the vessel. The vessel is inverted by 180degrees to be upside down at the stage at which the copper or the copperalloy is melted to arrive at the predetermined temperature. Accordingly,the copper or the copper alloy is allowed to make contact with theporous sintered compact. A high pressure is applied in the vessel sothat the porous sintered compact is impregnated with the copper or thecopper alloy.

In another preferred embodiment, the impregnating step may comprise thesteps of placing the metal having been previously melted and the poroussintered compact into an identical vessel, arranging the molten metal ata lower portion of the vessel, and then allowing the vessel to be in anegative pressure state or in an ordinary pressure state therein;inverting the vessel at a stage at which the molten metal arrives at apredetermined temperature to immerse the porous sintered compact in themolten metal in the vessel; and impregnating the porous sintered compactwith the molten metal by introducing an impregnating gas into the vesselto apply a pressure in the vessel.

That is, the previously melted copper or the copper alloy is placed intothe vessel in which the porous sintered compact is installed, and thevessel is inverted by 180 degrees to be upside down at the stage atwhich the contents in the vessel arrive at the predeterminedtemperature. Accordingly, the copper or the copper alloy is allowed tomake contact with the porous sintered compact. A high pressure isapplied in the vessel so that the porous sintered compact is impregnatedwith the copper or the copper alloy.

The applied pressure is not less than 10 kgf/cm² and not more than 1000kgf/cm², preferably not less than 50 kgf/cm² and not more than 200kgf/cm², and more preferably not less than 100 kgf/cm² and not more than150 kgf/cm².

In this embodiment, the pressure is applied for a period of time of notless than 1 minute and not more than 30 minutes, and desirably not lessthan 2 minutes and not more than 10 minutes.

The predetermined temperature is a temperature which is higher than amelting point of the copper or the copper alloy for impregnationtherewith by 30° C. to 250° C., and preferably the predeterminedtemperature is a temperature which is higher than the melting point by50° C. to 200° C. In this embodiment, it is preferable that the copperor the copper alloy for impregnating the porous sintered compacttherewith is heated in vacuum of not more than 1×10⁻³ Torr.

It is desirable that the porous sintered compact includes pores not lessthan 90% of which have an average diameter of 0.5 μm to 50 μm, having aporosity of 20% by volume to 70% by volume.

In a preferred embodiment, the porous sintered compact is previouslyplated with Ni in an amount of 1 to 10% by volume. In this embodiment,the wettability between the porous sintered compact and the copper orthe copper alloy is improved, and it is possible to realize impregnationat a low pressure. The amount of the Ni plating is desirably 3 to 5% byvolume. The Ni plating referred to herein includes, for example, Ni-Pplating and Ni-B plating.

It is also preferable that the porous sintered compact is previouslyimpregnated with 1 to 10% by volume of Si. In this embodiment, thewettability between the porous sintered compact and the copper or thecopper alloy is improved in the same manner as the case of theapplication of Ni plating described above, and it is possible to realizeimpregnation at a low pressure. The amount of the impregnation of Si isdesirably 3 to 5% by volume.

In relation to the Ni plating previously applied by 1 to 10% by volumeto the porous sintered compact, or the previous impregnation of Si by 1to 10% by volume, it is also preferable that the porous sintered compactis previously plated with palladium. In this embodiment, in addition tothe palladium plating, it is also possible to apply composite platingtogether with Ni and Si.

In still another preferred embodiment, the cooling step may comprise thesteps of inverting the vessel to separate the porous sintered compactafter the impregnation from the remaining molten metal not subjected tothe impregnation; and venting the impregnating gas from the vessel toquickly introduce a cooling gas so that the inside of the vessel iscooled. Alternatively, the cooling step may comprise the steps ofinverting the vessel to separate the porous sintered compact after theimpregnation from the remaining molten metal not subjected to theimpregnation; and allowing the vessel to make contact with a chill blockso that the inside of the vessel is cooled.

The cooling step is preferably performed at a cooling rate of not lessthan -400° C./hour from the temperature during the impregnation to 800°C., and more preferably not less than -800° C./hour.

The applied pressure is a pressure necessary to completely impregnatethe open pores of the porous sintered compact with the copper or thecopper alloy. In this process, if the open pores, which are notimpregnated with the copper or the copper alloy, remain in the poroussintered compact, the heat conductivity is markedly inhibited.Therefore, it is necessary to apply a high pressure.

The pressure can be approximately estimated in accordance with theexpression of Washburn. However, the smaller the pore diameter is, thelarger the required force is. For example, the required pressure is 400kgf/cm² in the case of 0.1 μmφ, 40 kgf/cm² in the case of 1.0 μmφ, and 4kgf/cm² in the case of 10 μmφ respectively.

A reaction occurs between the porous sintered compact and the copper orthe copper alloy in the molten state. For example, when SiC is used asthe porous sintered compact, SiC is decomposed into Si and C, and theoriginal function is not exhibited. For this reason, it is necessary toshorten the period of time during which SiC makes direct contact with Cuin the molten state. According to the production method concerning thepresent invention (the production method as defined in claim 14, 15, or20), it is possible to shorten the contact time between SiC and Cu.Accordingly, it is possible to avoid the decomposition reaction of SiCas described above beforehand.

The wettability is poor between SiC and the copper or the copper alloy.Therefore, it is necessary to apply the high pressure in order tosufficiently perform the impregnation with the copper or the copperalloy. According to the production method concerning the presentinvention (the production method as defined in claim 20 or 21), the poresurface of SiC is modified in quality to give good wettability betweenSiC and Cu. Accordingly, it is possible to impregnate finer pores withthe copper or the copper alloy at a lower pressure.

In still another preferred embodiment, the impregnating step maycomprise the steps of placing the porous sintered compact and the metalin a negative pressure state or in an ordinary pressure state whilemaking no contact with each other; heating the porous sintered compactand the metal to the predetermined temperature at the negative pressureor at the ordinary pressure to melt the metal; allowing the molten metalto be in a pressure-applied state; and allowing the molten metal at theapplied pressure to quickly make contact with the porous sinteredcompact at the negative pressure or at the ordinary pressure andallowing them to be in a pressure-applied state so that the poroussintered compact is impregnated with the molten metal at the appliedpressure; and the cooling step may comprise the step of cooling theporous sintered compact impregnated with the molten metal at the appliedpressure.

In this embodiment, the porous sintered compact and the metal are heatedwhile performing sufficient deaeration to melt the metal, followed bymaking quick contact and giving the pressure-applied state. Further, thepressure-applied state is maintained until completion of the coolingoperation. Thus, it is possible to efficiently impregnate the poroussintered compact with the molten metal.

In the production method as described above, it is preferable that bothof the porous sintered compact and the molten metal, which are heatedand treated while making no contact with each other at the negativepressure or at the ordinary pressure, are placed in the pressure-appliedstate, and then they are allowed to quickly make contact with each otherso that the porous sintered compact is impregnated with the metal.

Accordingly, the porous sintered compact is allowed to be in thepressure-applied state together with the molten metal, followed byperforming the contact and impregnating operations. Thus, it is possibleto minimize the pressure drop which would be caused upon the contact ofthe both, and it is possible to well maintain the pressure-applied stateduring the impregnating operation.

In still another preferred embodiment, the impregnating step maycomprise the steps of arranging the porous sintered compact and themetal respectively in upper and lower chambers of an identical vesselcomparted to have the two chambers by a porous filter, and tightlysealing the vessel so that the respective chambers are in a negativepressure state or in an ordinary pressure state; heating both of theupper and lower chambers at the negative pressure or at the ordinarypressure to a predetermined temperature so that the metal is melted;allowing only the upper chamber to be in a pressure-applied state; andallowing the molten metal in the upper chamber at the applied pressureto permeate through the porous filter to the lower chamber so that themolten metal quickly makes contact with the porous sintered compact atthe negative pressure or at the ordinary pressure, followed by allowingthe lower chamber to be in a pressure-applied state so that the poroussintered compact at the applied pressure is impregnated with the moltenmetal; and the cooling step may comprise the step of cooling the poroussintered compact impregnated with the molten metal in the lower chamberin the pressure-applied state.

In this embodiment, the upper chamber arranged with the metal and thelower chamber arranged with the porous sintered compact can beindependently subjected to pressure control by using the porous filter.Accordingly, it is possible to quickly reduce or apply the pressure byusing a predetermined pressure control mechanism.

The porous sintered compact in the lower chamber can be deaerated whilemaintaining it in the negative pressure state or in the ordinarypressure state immediately before the impregnation of the molten metal.Further, the contact and impregnating operations for the molten metaland the porous sintered compact can be easily performed in accordancewith the pressure control effected by the aid of the porous filter. Inthis process, the molten metal can be quickly treated with the filter,because of the difference in pressure which is provided beforehandbetween the both chambers.

The material for the porous filter is not specifically limited providedthat the material has a porous property of a degree so that the moltenmetal makes no permeation at the ordinary pressure and the molten metalmakes permeation at the applied pressure. Those preferably usable as thematerial include, for example, carbon cloth, punching metal composed ofstainless steel, and alumina cloth.

In still another preferred embodiment, the impregnating step maycomprise the steps of arranging the porous sintered compact and themetal respectively in upper and lower chambers of an identical vesselcomparted to have the two chambers by a porous filter, and tightlysealing the vessel so that the respective chambers are in a negativepressure state or in an ordinary pressure state; heating both of theupper and lower chambers at the negative pressure or at the ordinarypressure to a predetermined temperature so that the metal is melted;allowing both of the upper and lower chambers to be in apressure-applied state; and raising a pressure of the pressure-appliedupper chamber to be higher than a pressure of the lower chamber, andallowing the molten metal to permeate through the porous filter to thelower chamber so that the molten metal quickly makes contact with theporous sintered compact, and then the porous sintered compact at theapplied pressure is impregnated with the molten metal; and the coolingstep may comprise the step of cooling the porous sintered compactimpregnated with the molten metal in the lower chamber at the appliedpressure.

In this embodiment, the porous sintered compact is allowed to be in thepressure-applied state together with the molten metal, followed byperforming the contact and impregnating operations. Thus, it is possibleto minimize the pressure drop which would be caused upon the contact ofthe both, and it is possible to well maintain the pressure-applied stateduring the impregnating operation.

In the present invention, when the porous sintered compact to serve asthe base material is treated and impregnated with the metal containingat least copper, the step of providing the pressure-applied state may beperformed by means of a press treatment effected in both upward anddownward directions, and the cooling step may be performed by means ofan indirect cooling treatment effected in the vicinity of the lowerchamber.

According to the production method concerning the present invention, itis possible to more quickly perform the pressure control, and it ispossible to well maintain the pressure-applied state during theimpregnating operation.

In still another aspect, the present invention provides a method forproducing a composite material for heat sinks for semiconductor devices,the method comprising an impregnating step of allowing a porous sinteredcompact to serve as a base material to make contact with a metalcontaining at least copper at a negative pressure or at an ordinarypressure, performing a heating treatment to melt the metal, and thenquickly impregnating the porous sintered compact with the metal in apressure-applied state; and a cooling step of cooling at least theporous sintered compact impregnated with the metal.

Accordingly, the treatment for impregnating the porous sintered compactwith the copper or the copper alloy, which is generally considered to bedifficult, can be performed with ease. Moreover, it is possible toimprove the rate of impregnation of the copper or the copper alloy intothe porous sintered compact. As a result, it is possible to improve theproductivity of the heat sink which has characteristics adapted to thebalance between the coefficient of thermal expansion and the coefficientof thermal conductivity demanded for actual electronic parts or the like(including semiconductor devices).

In a preferred embodiment, the impregnating step may comprise the stepsof placing the porous sintered compact and the metal in a negativepressure state or in an ordinary pressure state while making contactwith each other; heating the porous sintered compact and the metal to apredetermined temperature at the negative pressure or at the ordinarypressure to melt the metal; allowing the molten metal to be in apressure-applied state; and allowing the molten metal at the appliedpressure to quickly make contact with the porous sintered compact at thenegative pressure or at the ordinary pressure and allowing them to be ina pressure-applied state so that the porous sintered compact isimpregnated with the molten metal at the applied pressure; and thecooling step may comprise the step of cooling the porous sinteredcompact impregnated with the molten metal at the applied pressure.

As described above, according to the production method concerning thepresent invention, it is possible to suppress the expansion to be at avalue which is lower than the thermal expansion amount (theoreticalvalue) determined by the ratio between the porous sintered compact andthe copper or the copper alloy with which the porous sintered compact isimpregnated. The coefficient of thermal expansion is approximatelycoincident with those of, for example, the ceramic substrate and thesemiconductor substrate (silicon, GaAs). Thus, it is possible to obtainthe material for heat sinks having good thermal conductivity.

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which a preferredembodiment of the present invention is shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of the use of a heat sink constructed byusing a composite material prepared in accordance with an embodiment ofthe present invention.

FIG. 2 shows a perspective view conceptually illustrating the structureof a composite material prepared in accordance with an embodiment of thepresent invention.

FIG. 3 shows a table illustrating the difference in coefficient ofthermal conductivity and coefficient of thermal expansion obtained byappropriately changing the porosity, the pore diameter, and the poredistribution of SiC.

FIG. 4 shows characteristic curves illustrating the coefficient ofthermal conductivity-coefficient of thermal expansion characteristic ofvarious composite materials.

FIG. 5A shows, with partial cutaway, a front face of a high-pressurevessel.

FIG. 5B shows, with partial cutaway, a side face of the high-pressurevessel.

FIG. 6 shows a block diagram illustrating the steps of a productionmethod concerning a first embodiment.

FIG. 7 shows a table illustrating the difference in reaction situationof SiC/Cu and impregnation situation of Cu obtained by appropriatelychanging the porosity of SiC, the pore diameter, the presence or absenceof Ni plating, the presence or absence of Si impregnation, theimpregnation temperature, the applied pressure, the pressure-applyingtime, and the cooling rate.

FIG. 8 shows a block diagram illustrating the impregnating stepconcerning a first modified embodiment.

FIG. 9 shows a block diagram illustrating the impregnating stepconcerning a second modified embodiment.

FIG. 10 shows schematic construction of a hot press furnace used for aproduction method concerning a second embodiment.

FIG. 11 shows a block diagram illustrating the steps of a productionmethod concerning a second embodiment.

FIG. 12A shows a plan view illustrating a split-type packing member.

FIG. 12B shows a cross-sectional view taken along a line A--A shown inFIG. 12A.

FIG. 13 shows schematic construction of another embodiment of the hotpress furnace used for the production method concerning the secondembodiment.

FIG. 14 shows a hot press furnace used for a modified embodiment of theproduction method concerning the second embodiment.

FIG. 15 shows a block diagram illustrating the steps of a modifiedembodiment of the production method concerning the second embodiment.

FIG. 16 shows construction of a hot press furnace used for a productionmethod concerning a third embodiment.

FIG. 17 shows a block diagram illustrating the steps of the productionmethod concerning the third embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Illustrative embodiments of the composite material for heat sinks forsemiconductor devices and the method for producing the same according tothe present invention (hereinafter simply referred to as "compositematerial according to the embodiment" and "production method accordingto the embodiment") will be explained below with reference to FIGS. 1 to17.

At first, as shown in FIG. 1, the heat sink 10, which is constructed bythe composite material according to the embodiment of the presentinvention, is embedded in an opening 16 formed in an upper surface of apackage 14 of a semiconductor device 12. The heat sink 10 is installedsuch that its surface contacts with a cooling fin 18 secured to an uppersection of the semiconductor device 12.

Accordingly, the heat, which is released from the semiconductor element(chip) mounted in the semiconductor device 12, is efficientlytransmitted to the cooling fin 18 through the heat sink 10.

The composite material according to the embodiment of the presentinvention is constructed by impregnating, with copper or copper alloy, aporous sintered compact obtained by pre-calcinating a porous body havinga coefficient of thermal expansion which is lower than the coefficientof thermal expansion of copper so that a network structure is formed.Specifically, for example, as shown in FIG. 2, the composite material isconstructed such that open pores (open pore portions) of a poroussintered compact 20 composed of SiC are impregnated with the copper orcopper alloy 22. In the following description, the porous sinteredcompact composed of SiC will be simply referred to as "SiC".

According to the construction described above, it is possible tosuppress the expansion to have a value which is lower than the thermalexpansion (theoretical value) determined by the ratio between SiC andthe copper or copper alloy 22 for impregnating SiC therewith asdescribed above later on. The coefficient of thermal expansion isapproximately coincident with those of, for example, ceramic substratesand semiconductor substrates (silicon, GaAs) to be used as the substratefor electronic parts or the like. Thus, it is possible to obtain thematerial for heat sinks having good thermal conductivity. Specifically,it is possible to obtain the material for heat sinks in which theaverage coefficient of thermal expansion in a range from roomtemperature to 200° C. is 4.0×10⁻⁶ /° C. to 9.0×10⁻⁶ /° C., and thecoefficient of thermal conductivity is not less than 180 W/mK (roomtemperature).

In the present invention, it is desirable that the porosity of SiC(approximately the same as the impregnation rate of the copper or copperalloy 22) is 20% by volume to 70% by volume. If the porosity is lessthan 20% by volume, it is impossible to obtain the coefficient ofthermal expansion of 180 W/mK (room temperature). If the porosityexceeds 70% by volume, then the strength of SiC is lowered, and it isimpossible to suppress the coefficient of thermal expansion to be lessthan 9.0×10⁻⁶ /° C.

It is desirable that the value of the average open pore diameter (porediameter) of SiC is 0.5 to 50 μm. If the pore diameter is less than 0.5μm, then it is difficult to impregnate the open pores with the copper orcopper alloy 22, and the coefficient of thermal conductivity is lowered.On the other hand, if the pore diameter exceeds 50 μm, then the strengthof SiC is lowered, and it is impossible to suppress the coefficient ofthermal expansion to be low.

It is preferable that the distribution (pore distribution) in relationto the average open pores of SiC is distributed by not less than 90% ina range of 0.5 to 50 μm. If the pores of 0.5 to 50 μm are notdistributed by not less than 90%, the open pores, which are notimpregnated with the copper or copper alloy 22, are increased.Consequently, the coefficient of thermal conductivity is decreased.

The porosity, the pore diameter, and the pore distribution were measuredby using an automatic porosimeter (trade name: Autopore 9200) producedby Simadzu Corporation.

It is preferable that bending strength of SiC is not less than 10 MPa,desirably not less than 20 MPa, and more desirably not less than 30 MPa,because of the following reason. That is, if the bending strength islower than 10 MPa, a problem arises in that the coefficient of thermalexpansion is increased.

In general, when commercially available pure copper is used as thecopper, a good composite material having a high coefficient of thermalconductivity is obtained. However, the obtained composite material isnot excellent in wettability with respect to the porous sintered compact(especially SiC), and the open pores which are not impregnated withcopper tend to remain. Therefore, it is desirable to improve theimpregnation rate by adding, for example, Be, Al, Si, Mg, Ti, and Ni. Inthis case, the component of copper may contain one or more species ofBe, Al, Si, Mg, Ti, and Ni in a range up to 1%, further containing gascomponents and inevitable impurities such as Ag, Cd, Zn, Au, Pd, In, Ga,Pt, Cr, Ge, Rh, Sb, Ir, Co, As, Zr, Fe, Sn, Mn, P, and Pb. However, ifthe additive is added in an amount more than 1%, then the coefficient ofthermal conductivity is greatly decreased, and it is impossible toobtain the effect which would be otherwise obtained by the addition.

An illustrative experiment will now be described. In this illustrativeexperiment, observation was made for the difference in coefficient ofthermal conductivity and coefficient of thermal expansion when theporosity, the pore diameter, and the pore distribution of SiC wereappropriately changed. Obtained experimental results are shown in atable in FIG. 3.

In FIG. 3, Examples 1 to 8 represent cases in which copper was used asthe impregnating material, while changing the porosity, the porediameter, and the pore distribution within the predetermined rangesrespectively. Example 9 represents a case in which a copper alloycontaining 0.5% by weight of Be and the balance of Cu was used as theimpregnating material, while allowing the porosity, the pore diameter,and the pore distribution to be within the predetermined rangesrespectively. Comparative Example 1 represents a case in which copperwas used as the impregnating material, and the porosity was deviatedfrom the predetermined range. Comparative Example 2 represents a case inwhich copper was used as the impregnating material, and the porosity andthe pore distribution were deviated from the predetermined rangesrespectively. Comparative Example 3 represents a case in which copperwas used as the impregnating material, and the pore diameter and thepore distribution were deviated from the predetermined rangesrespectively.

According to the experimental results, any one of Examples 1 to 9satisfies the coefficient of thermal conductivity=180 W/mK (roomtemperature) or more, and the coefficient of thermal expansion=4.0×10⁻⁶/° C. to 9.0×10⁻⁶ /° C. The coefficient of thermal expansion is anaverage value of those obtained at room temperature to 200° C.

On the other hand, in Comparative Example 1, the porosity has a valuelower than the predetermined range. Therefore, the impregnation rate ofcopper is lowered, in accordance with which the coefficient of thermalconductivity is 165 W/mK which is also low. In Comparative Example 2,the porosity has a value higher than the predetermined range. Therefore,the impregnation rate of copper is increased, and the coefficient ofthermal conductivity is 325 W/mK which is also high. However, thestrength of SiC is lowered in a degree corresponding thereto, and thecoefficient of thermal expansion is 12.4×10⁻⁶ /° C. which is high.

When the results obtained in Examples 1 to 8 are plotted while givingthe coefficient of thermal conductivity along the horizontal axis andgiving the coefficient of thermal expansion along the vertical axis, thecharacteristic obtained when SiC is impregnated with copper is assumedto be represented by a characteristic curve as shown by a curve "a" inFIG. 4. In FIG. 4, a curve "b" indicates theoretical values obtainedwhen SiC and aluminum are subjected to powder shaping, a curve "c"indicates theoretical values obtained when SiC and copper are subjectedto powder shaping, and a curve "d" indicates actually measured valuesobtained when SiC is impregnated with aluminum.

In FIG. 4, a region A indicated by an ellipse depicted by a dashed linerepresents a characteristic region of Cu-W which has been hitherto usedas the material for heat sinks. As shown in FIG. 3, it is understoodthat Examples 6 to 8 have their characteristics which are superior tothe characteristic of Cu-W described above, because of the followingreason. That is, in any one of Examples 6 to 8, the range of thecoefficient of thermal expansion is approximately the same as that ofCu-W, and the range of the coefficient of thermal conductivity is higherthan that of Cu-W, Examples 6 to 8 being included in an optimumcharacteristic range (range indicated by a rectangle depicted by brokenlines) B as for the material for heat sinks. Therefore, thecharacteristics of the composite material can be concentrated into theoptimum characteristic range as for the material for heat sinks byoptimizing the porosity, the pore diameter, and the pore distribution ofSiC.

Next, the production methods according to the first and secondembodiments will be explained with reference to FIGS. 5 to 15. Theproduction methods according to the first and second embodimentscomprise the steps which may be roughly classified into the impregnatingstep and the cooling step. In the impregnating step, SiC to serve as thebase material and the copper or copper alloy are heated while making nocontact with each other, and the both are allowed to make contact at thestage of arrival at a predetermined temperature to immediately apply ahigh pressure so that SiC is impregnated with the copper or copperalloy. In the cooling step, SiC impregnated with the copper or copperalloy is cooled.

At first, as specifically exemplified in FIGS. 5A and 5B, the productionmethod according to the first embodiment is carried out by using ahigh-pressure vessel 30. The high-pressure vessel 30 is provided withrotary shafts 38 at approximately central portions of both side plates34, 36 of a hollow rectangular parallelepiped housing 32 respectively.The housing 32 itself is rotatable about the rotary shafts 38 as acenter.

A refractory vessel 40 and a heater 42 for heating the refractory vessel40 are provided in the housing 32. The refractory vessel 40 has a hollowrectangular parallelepiped configuration having a hollow section 44therein. An opening 46, which communicates with the hollow section 44,is provided at a central portion in the vertical direction at one sidesurface. The hollow section 44 includes one hollow section (hereinafterreferred to as "first chamber 44a") comparted with respect to thecentral opening 46 as a center. Ingot of copper or copper alloy 22 ormolten metal of copper or copper alloy 22 to be used as an impregnatingmaterial is accommodated in the first chamber 44a. A plurality pieces ofSiC 20 to be used as a material to be impregnated are attached to theother hollow section (hereinafter referred to as "second chamber 44b").A support mechanism for SiC 20 is provided so that SiC 20 does not fallsoff even when the second chamber 44b is located upward. The heater 42has a structure which is not destroyed even at a high pressure of 100kgf/cm².

The high-pressure vessel 30 is provided with a suction pipe 48 forvacuum suction, and an inlet pipe 50 and an outlet pipe 52 for a gas forapplying a high pressure and a gas for cooling.

Next, the impregnating step and the cooling step, which are performed byusing the high-pressure vessel 30, will be explained with reference toFIG. 6. The impregnating step is performed by executing the followingsteps.

At first, the high-pressure vessel 30 is allowed to be in an initialstate, and the first chamber 44a of the refractory vessel 40, which isprovided inside the high-pressure vessel 30, is disposed downward (stepS1).

Subsequently, SiC 20 and the ingot of copper or copper alloy 22 areplaced in the refractory vessel 40 of the high-pressure vessel 30. Theingot of copper or copper alloy 22 is arranged in the first chamber 44aof the refractory vessel 40, and SiC 20 is set in the second chamber 44b(step S2). After that, the high-pressure vessel 30 (and the refractoryvessel 40) is tightly sealed, and then vacuum suction is performed forthe inside of the high-pressure vessel 30 through the suction pipe 48 sothat the inside of the high-pressure vessel 30 is in a negative pressurestate (step S3).

After that, an electric power is applied to the heater 42 to heat andmelt the copper or copper alloy 22 in the first chamber 44a (step S4).In the following explanation, the copper or copper alloy 22, which hasbeen heated and melted, will be conveniently referred to as "moltencopper".

Subsequently, the high-pressure vessel 30 is inverted by 180 degrees ata stage at which the molten copper in the first chamber 44a arrives at apredetermined temperature (step S5). This inverting operation allows thefirst chamber 44a to be disposed upward. Accordingly, the molten copperin the first chamber 44a falls downwardly by its self-weight toward thesecond chamber 44b disposed thereunder. At this stage, a state is givenin which SiC 20 is immersed in the molten copper.

After that, an impregnating gas is introduced into the high-pressurevessel 30 through the gas inlet pipe 50 to apply a pressure to theinside of the high-pressure vessel 30 (step S6). This pressure-applyingtreatment allows open pores of SiC 20 to be impregnated with the moltencopper.

The procedure immediately proceeds to the cooling step upon completionof the impregnating step. In the cooling step, at first, thehigh-pressure vessel 30 is inverted by 180 degrees again (step S7). Thisinverting operation allows the first chamber 44a to be disposeddownward. Accordingly, the molten copper in the second chamber 44b fallsdownwardly toward the first chamber 44a again. The open pores of SiC 20have been impregnated with a part of the molten copper as a result ofthe pressure-applying treatment (impregnating treatment) effected in thestep S6. Therefore, the molten copper, which falls downwardly toward theunderlying first chamber 44a, is remaining molten copper not subjectedto the impregnation of SiC 20. At a stage at which the remaining moltencopper has fallen downwardly toward the first chamber 44a, SiC 20impregnated with the molten copper remains in the second chamber 44b.

Subsequently, the impregnating gas in the high-pressure vessel 30 isdischarged through the gas outlet pipe 52, simultaneously with which acooling gas is introduced into the high-pressure vessel 30 through thegas inlet pipe 50 (step S8). The discharge of the impregnating gas andthe introduction of the cooling gas allow the cooling gas to thoroughlycirculate through the inside of the high-pressure vessel 30, and thusthe high-pressure vessel 30 is quickly cooled. In accordance with thequick cooling, the molten copper impregnated into SiC 20 is quicklysolidified into mass or lump of the copper or copper alloy 22, and thevolume is expanded. Thus, the copper or copper alloy 22 subjected to theimpregnation is tightly retained by SiC 20.

Another cooling step is available as shown in a frame depicted by dashedlines in FIG. 6. That is, the high-pressure vessel 30 or SiC 20impregnated with the molten copper is allowed to make contact with achill block at a stage at which the treatment in the step S7 has beencompleted (step S9). SiC 20 is quickly cooled by means of the contactwith the chill block. The cooling process may be carried out whilecooling the chill block with water, or the cooling process may becarried out while installing the chill block at a place separated fromthe heating body. Especially, it is preferable to perform the cooling inconsideration of the risering efficiency.

As described above, the treatment for impregnating SiC 20 with thecopper or copper alloy 22, which is generally considered to bedifficult, can be easily performed by executing the impregnating stepand the cooling step. Moreover, it is possible to improve theimpregnation rate of the copper or copper alloy 22 into the SiC 20.Therefore, it possible to improve the productivity of the heat sink 10which has characteristics adapted to the balance between the coefficientof thermal expansion and the coefficient of thermal conductivitydemanded for actual electronic parts or the like (includingsemiconductor devices), i.e., the coefficient of thermal expansion of4.0×10⁻⁶ /° C. to 9.0×10⁻⁶ /° C. from room temperature to 200° C., andthe coefficient of thermal conductivity of 180 W/mK (room temperature)or more.

In the step S4, when the electric power is applied to the heater 42 toheat and melt the copper or copper alloy 22 in the first chamber 44a, itis desirable that the predetermined temperature (heating temperature) toproceed to the step S5 is a temperature higher than the melting point ofthe copper or copper alloy 22 by 30° C. to 250° C., and preferably atemperature higher than the melting point by 50° C. to 200° C. In thisprocess, it is preferable that the inside of the high-pressure vessel 30is in vacuum of not more than 1×10⁻³ Torr.

In the step S6, the pressure, which is applied to the high-pressurevessel 30 by introducing the impregnating gas into the high-pressurevessel 30, is not less than 10 kgf/cm² and not more than 1000 kgf/cm².In this procedure, the pressure is preferably not less than 50 kgf /cm²and not more than 200 kgf/cm², and more preferably not less than 100kgf/cm² and not more than 150 kgf/cm².

It is desirable that the pressure is preferably applied to thehigh-pressure vessel 30 for a period of time of not less than 1 minuteand not more than 30 minutes, and more preferably not less than 2minutes and not more than 10 minutes.

As described above, it is desirable for the pores of SiC 20 that thosehaving an average diameter of 5 μm to 50 μm exist in an amount of 90% ormore, and the porosity is 20% by volume to 70% by volume.

It is preferable that SiC 20 is previously plated with Ni of 1 to 10% byvolume, and desirably 3 to 5% by volume, in order to improve thewettability between SiC 20 and the copper or copper alloy 22. In thiscase, it is possible to realize impregnation by using a low pressure.The Ni plating referred to herein includes, for example, Ni-P platingand Ni-B plating.

In order to improve the wettability between SiC 20 and the copper orcopper alloy 22, it is preferable that SiC 20 is previously impregnatedwith 1 to 10% by volume of Si, and more desirably 3 to 5% by volume ofSi. In this case, it is also possible to realize impregnation by using alow pressure.

In relation to the Ni plating previously applied by 1 to 10% by volumeto SiC 20, or the previous impregnation of Si by 1 to 10% by volume, itis also preferable that SiC 20 is previously plated with palladium. Inthis case, in addition to the palladium plating, it is also possible toapply composite plating together with Ni and Si.

On the other hand, it is desirable that the cooling step is preferablyperformed at a cooling rate of not less than -400° C./hour in a rangefrom the temperature during the impregnation to 800° C., and morepreferably not less than -800° C./hour.

In the step S6, the pressure applied to the high-pressure vessel 30 is apressure necessary to completely impregnate the open pores of SiC 20with the copper or copper alloy 22. In this process, if open pores,which are not impregnated with the copper or copper alloy 22, remain inSiC 20, the heat conductivity is markedly inhibited. Therefore, it isnecessary to apply a high pressure.

The pressure can be approximately estimated in accordance with theexpression of Washburn. However, the smaller the pore diameter is, thelarger the required force is. For example, the required pressure is 400kgf/cm² in the case of 0.1 μmφ, 40 kgf/cm² in the case of 1.0 μmφ, and 4kgf/cm² in the case of 10 μmφ respectively.

A reaction occurs between SiC 20 and the copper or copper alloy 22 at ahigh temperature. SiC 20 is decomposed into Si and C, and the originalfunction is not exhibited. For this reason, it is necessary to shortenthe period of time during which SiC 20 makes direct contact with thecopper or copper alloy 22 at the high temperature. It is possible toshorten the contact time between SiC 20 and the copper or copper alloy22 by satisfying the first treatment condition (pressure applied to thehigh-pressure vessel 30=not less than 10 kgf/cm² and not more than 1000kgf/cm²), the second treatment condition (heatingtemperature=temperature higher than the melting point of the copper orcopper alloy 22 by 30° C. to 250° C.), and the third treatment condition(1 to 10% by volume of Ni plating is previously applied to SiC 20).Accordingly, it is possible to avoid the decomposition reaction of SiC20 as described above beforehand.

The wettability is poor between SiC 20 and the copper or copper alloy22. Therefore, it is necessary to apply the high pressure in order tosufficiently perform the impregnation with the copper or copper alloy22. The pore surface of SiC 20 is modified in quality to give goodwettability between SiC 20 and the copper or copper alloy 22 byeffecting the third treatment condition (1 to 10% by volume of Niplating is previously applied to SiC 20) or the fourth treatmentcondition (SiC 20 is previously impregnated with 1 to 10% by volume ofSi). Accordingly, it is possible to impregnate finer pores with thecopper or copper alloy 22 at a lower pressure.

An illustrative experiment will now be described. In this illustrativeexperiment, observation was made for the difference in reactionsituation of SiC/Cu and impregnation situation of Cu obtained byappropriately changing the porosity of SiC 20, the pore diameter, thepresence or absence of Ni plating, the presence or absence of Siimpregnation, the impregnation temperature, the applied pressure, thepressure-applying time, and the cooling rate. Obtained experimentalresults are shown in a table in FIG. 7. In FIG. 7, the situation of thereaction of SiC/Cu was determined by the thickness (average value) ofthe reaction layer formed between SiC and Cu. The determinativecondition in the experiment is as follows. The basis of thedeterminative condition is the fact that when the reaction layer of notless than 5 μm is produced between SiC and Cu, the thermal transferbetween SiC and Cu is deteriorated, resulting in decrease in thermalconductivity when the obtained material is used as a composite materialfor heat sinks for semiconductor devices.

Thickness (average) of reaction layer is not more than 1 μm→"noreaction";

Thickness (average) of reaction layer is more than 1 μm and not morethan 5 μm→"less reaction";

Thickness (average) of reaction layer is more than 5 μm→"much reaction".

According to the experimental results, as for any one of those whichsatisfy the predetermined ranges of the porosity of SiC 20, the porediameter, the impregnation temperature, the applied pressure, thepressure-applying time, and the cooling rate (Samples 3, 7, 8, 11, and12), the reaction situation of SiC/Cu is "no reaction", and theimpregnation situation of Cu is good. Thus, good results are obtained.

Concerning Samples 3, 7, 8, 11, and 12 described above, the Ni platingor the Si impregnation is performed for Samples 3, 7, 11, and 12.Therefore, the wettability with respect to Cu is good, and the goodresults are obtained as described above even when the pressure-applyingtime is short. As for Sample 8, the Ni plating and the Si impregnationare not performed. However, the pressure-applying time is successfullyshortened by increasing the applied pressure. Thus, the good results areobtained as described above.

On the other hand, as for Samples 1, 5, and 9 in which the appliedpressure is 8 kgf/cm² which is lower than the predetermined range, theimpregnation situation of Cu is insufficient for any of them. ConcerningSamples 1, 5, and 9 described above, the reaction situation of SiC/Cu is"much reaction" for those having the long pressure-applying time(Samples 1 and 5).

As for Sample 6, the impregnation situation is insufficient, althoughthe reaction situation of SiC/Cu is "less reaction", probably becausethe porosity and the pore diameter do not satisfy the predeterminedranges respectively. As for Sample 14, the reaction situation of SiC/Cuis "much reaction", although the impregnation situation is good,probably because the pore diameter is larger than the predeterminedrange, and the pressure-applying time is relatively long.

Incidentally, for example, a sintered compact of Si-SiC containing amajor phase comprising 2 to 25% by weight of Si and 75 to 98% by weightof SiC can be used as SiC impregnated with Si (Si-SiC sintered compact)which has good wettability with respect to the copper or copper alloy22. In order to obtain the Si-SiC sintered compact, it is preferablethat Al impurity is controlled to be not more than 0.2 part by weightand SiO₂ is controlled to be not more than 3.0 parts by weight withrespect to 100 parts by weight of the major phase respectively, and thatthe entire amount of impurities is controlled to be 0.4 to 4.2 parts byweight with respect to 100 parts by weight of the major phase.

Specifically, a method for producing the Si-SiC sintered compact will bebriefly described. At first, as for the material for shaping, a rowmaterial containing SiC powder, carbon powder, organic binder, and wateror organic solvent is used.

The material for shaping is mixed and kneaded, and it is shaped into apredetermined shape to prepare a compact. Subsequently, the compact isplaced in an atmosphere of inert gas at a reduced-pressure or in vacuumin a metallic silicon atmosphere. The compact is impregnated with themetallic silicon to produce the Si-SiC sintered compact.

Any of the press shaping, the casting shaping, and the extrusion moldingmay be available as the shaping method. However, it is preferable to usethe press shaping in view of mass production performance. The hydraulicpress is preferably used as a method for applying the pressure. In thiscase, the hydraulic press pressure is usually 50 to 2000 kg/cm².

Next, several modified embodiments concerning the impregnating step ofthe production method according to the first embodiment will beexplained with reference to FIGS. 8 and 9.

As shown in FIG. 8, in the impregnating step concerning the firstmodified embodiment, at first, the high-pressure vessel 30 is allowed tobe in an initial state. The first chamber 44a of the refractory chamber40 provided in the high-pressure vessel 30 is disposed downward (stepS101).

After that, SiC 20 is set in the second chamber 44b, and the previouslymelted copper or copper alloy (molten copper) 22 is poured into thefirst chamber 44a (step S102).

Subsequently, the high-pressure vessel 30 is inverted by 180 degrees ata stage at which the molten copper in the first chamber 44a arrives at apredetermined temperature (step S103). This inverting operation allowsthe molten copper in the first chamber 44a to fall downwardly toward theunderlying second chamber 44b. At this stage, a state is given in whichSiC 20 is immersed in the molten copper.

After that, the impregnating gas is introduced into the high-pressurevessel 30 through the gas inlet pipe 50 to apply the pressure to theinside of the high-pressure vessel 30 (step S104). Thispressure-applying treatment allows the open pores of SiC to beimpregnated with the molten copper.

Next, the impregnating step concerning the second modified embodimentwill be explained. The impregnating step concerning the second modifiedembodiment is based on the use of a high-pressure vessel 30 having apartition wall (not shown) composed of a porous ceramic member providedat an internal central portion of a refractory vessel 40 installed inthe high-pressure vessel 30. The inside of the refractory vessel 40 iscomparted by the partition wall into a first chamber 44a and a secondchamber 44b.

It is desirable that a porous ceramic member having a porosity of 40% to90% and a pore diameter of 0.5 mm to 3.0 mm is used as the partitionwall. More preferably, it is desirable to use a ceramic member having aporosity of 70% to 85% and a pore diameter of 1.0 mm to 2.0 mm.

In the impregnating step concerning the second modified embodiment, atfirst, as shown in FIG. 9, the high-pressure vessel is allowed to be inan initial state. The first chamber 44a of the refractory vessel 40provided in the high-pressure vessel is disposed downward, and thesecond chamber 44b is disposed upward (step S201).

After that, SiC 20 and ingot of copper or copper alloy 22 are placed inthe refractory vessel 40 of the high-pressure vessel 30. The ingot ofcopper or copper alloy 22 is disposed in the second chamber 44b locatedupward, and SiC 20 is set in the first chamber 44a located downward(step S202).

Subsequently, the high-pressure vessel 30 (and the refractory vessel 40)is tightly sealed, and then vacuum suction is performed for the insideof the high-pressure vessel 30 through the suction pipe 48 so that theinside of the high-pressure vessel 30 is in a negative pressure state(step S203).

After that, an electric power is applied to the heater 42 to heat andmelt the copper or copper alloy 22 in the second chamber 44b (stepS204). The impregnating gas is introduced into the high-pressure vessel30 through the gas inlet pipe 50 at a stage at which the molten copperarrives at a predetermined temperature to apply a pressure to the insideof the high-pressure vessel 30 (step S205). This pressure-applyingtreatment allows the molten copper in the second chamber 44b disposedupward to pass through the partition wall, and thus open pores of SiC 20in the first chamber 44a disposed downward are impregnated therewith.

Next, a production method according to a second embodiment will beexplained with reference to FIGS. 10 to 15.

Specifically, the production method according to the second embodimentis carried out by using a hot press furnace 60 as exemplified in FIG.10. The hot press furnace 60 includes, in a cylindrical housing 62, alower punch 64 which also serves as a base, a refractory vessel 66 whichhas an opening at its upper surface and which is fixed to the lowerpunch 64, an upper punch 68 which is movable frontwardly and rearwardlyfrom an upper position into the refractory vessel 66, and a heater 70for heating the refractory vessel 66. The hot press furnace 60 furthercomprises a suction pipe 72 for vacuum suction.

The refractory vessel 66 has a cylindrical shape having a hollow section74. The upper punch 68 is provided at its side surface with a flange 76for determining the stroke of the upper punch 68. The flange 76 has itslower surface which is attached with a packing 78 for making contactwith an upper circumferential surface of the refractory vessel 66 toprovide a tightly sealed state for the refractory vessel 66. On theother hand, the lower punch 64 includes, at its inside, a passage 80 forallowing a heating fluid for heating the inside of the refractory vessel66 and a cooling fluid for cooling the inside of the refractory vessel66 to flow therethrough.

The production method according to the second embodiment is carried outby executing the steps shown in FIG. 11.

At first, SiC 20, a filter 54 composed of porous ceramic, and ingot ofcopper or copper alloy 22 are introduced into the hollow section 74 ofthe refractory vessel 66 in this order from the bottom (step S301). Itis desirable that a porous ceramic member having a porosity of 40% to90% and a pore diameter of 0.5 mm to 3.0 mm is used as the filter 54.More preferably, it is desirable to use a ceramic member having aporosity of 70% to 85% and a pore diameter of 1.0 mm to 2.0 mm.

The filter 54 also functions as a partition wall for partitioning SiC 20from the ingot of copper or copper alloy 22 to allow the both to be in anon-contact state. A portion of the hollow section 74, to which theingot of copper or copper alloy 22 is set over the filter 54, may bedefined as an upper chamber 74a. A portion of the hollow section 74, towhich SiC 20 is set under the filter 54, may be defined as a lowerchamber 74b.

Next, after tightly sealing the refractory vessel 66, the inside of therefractory vessel 66 is subjected to vacuum suction through the suctionpipe 72 so that the inside of the both chambers 74a, 74b of therefractory vessel 66 is in a negative pressure state (step S302).

After that, an electric power is applied to the heater 70 to heat andmelt the copper or copper alloy 22 in the upper chamber 74a (step S303).During this process, the heating fluid may be allowed to flow throughthe passage 80 in the lower punch 64 together with the electric powerapplication to the heater 70 so that the inside of the refractory vessel66 is heated.

At a stage at which the melted matter (molten copper) of the copper orcopper alloy 22 in the upper chamber 74a arrives at a predeterminedtemperature, the upper punch 68 is moved downwardly to pressurize theinside of the upper chamber up to a predetermined pressure (step S304).At this time, the refractory vessel 66 is tightly sealed in accordancewith the contact and the mutual pressing effected between the uppercircumferential surface of the refractory vessel 66 and the packing 78attached to the flange 76 of the upper punch 68. Thus, it is possible toeffectively avoid the inconvenience that the molten copper containedtherein would be leaked out to the outside of the refractory vessel 66.

The melted matter (molten copper) of the copper or copper alloy 22 inthe upper chamber 74a, which has arrived at the predetermined pressure,is extruded by the pressure in the upper chamber 74a through the filter54 toward the lower chamber 74b, and it is introduced into the lowerchamber 74b, simultaneously with which SiC 20 installed in the lowerchamber 74b is impregnated therewith.

At a stage of arrival at the end point (point of time at which theimpregnation of the molten copper into SiC 20 is in a saturated state)previously set by time management, in turn, the cooling fluid is allowedto flow through the passage 80 in the lower punch 64 to cool therefractory vessel 66 from lower portions to upper portions (step S305).Thus, the molten copper impregnated into SiC 20 is solidified. Thepressure-applied state in the refractory vessel 66, which is effected bythe upper punch 68 and the lower punch 64, is maintained untilcompletion of the solidification.

At the point of time of completion of the solidification, SiC 20impregnated with the copper or copper alloy 22 is extracted from therefractory vessel 66 (step S306).

In this production method, SiC 20 and the copper or copper alloy 22 areheated while performing sufficient deaeration, and the copper or copperalloy 22 is melted, followed by the prompt contact with SiC 20 to givethe pressure-applied state for them. Further, the pressure-applied stateis maintained until completion of the cooling operation. Therefore, itis possible to efficiently impregnate SiC 20 with the copper or copperalloy 22. In the embodiment described above, the impregnating treatmentis performed at the negative pressure. However, the impregnatingtreatment may be performed at an ordinary pressure.

As described above, after both of the molten copper and SiC 20 areallowed to be in the pressure-applied state, they are allowed to makecontact with each other to perform the impregnating treatment.Accordingly, the pressure drop, which would occur upon the contact ofthe both, can be minimized. Thus, it is possible to well maintain thepressure-applied state during the impregnating treatment.

In the embodiment described above, the packing 78 is provided at thelower surface of the flange 76 of the upper punch 68 in order to avoidleakage of the molten copper. However, as shown by two-dot chain linesin FIG. 10, a packing 78 may be provided on the upper circumferentialsurface of the refractory vessel 66. Alternatively, as shown in FIG.12A, a packing member 102, which comprises two superimposed ring-shapedsplit-type packings 100, may be provided at a lower portion of the upperpunch 68 as shown in FIG. 13. In this arrangement, the molten copperenters a hollow section 104 of the packing member 102. Thus, thediameter of each of the split-type packings 100 is increased.Consequently, the upper chamber 74a is tightly sealed, and thus themolten copper is prevented from leakage.

Next, a modified embodiment of the production method according to thesecond embodiment will be explained with reference to FIGS. 14 and 15.Components or parts corresponding to those shown in FIG. 10 aredesignated by the same reference numerals, duplicate explanation ofwhich will be omitted.

As shown in FIG. 14, the production method concerning this modifiedembodiment is based on the use of a hot press furnace 60 comprising afilter member 110 composed of porous ceramic which is secured to acentral portion in the vertical direction of the hollow section 74 ofthe refractory vessel 66, and a door 112 which is attached to a sidesurface of the lower chamber 74b and which is freely capable of beingopened and closed. Therefore, the portion of the hollow section 74 ofthe refractory vessel 66, which is located higher than the filter member110, serves as the upper chamber 74a. The portion, which is locatedlower than the filter member 110, serves as the lower chamber 74b.Especially, as for the door 112 attached to the lower chamber 74b, astructure is adopted so that the lower chamber 74b is tightly sealedwhen the door 112 is closed.

The production method concerning the modified embodiment is carried outby executing the steps shown in FIG. 15.

At first, ingot of copper or copper alloy 22 is introduced into theupper chamber 74a of the refractory vessel 66. The door 112 of the lowerchamber 74b is opened, and SiC 20 is introduced into the lower chamber74b (S401).

Next, the door 112 is closed to tightly seal the lower chamber 74b, andthe hot press furnace 60 is tightly sealed. After that, the inside ofthe refractory vessel 66 is subjected to vacuum suction through thesuction pipe 72 so that the inside of the both chambers 74a, 74b of therefractory vessel 66 is in a negative pressure state (step S402).

After that, an electric power is applied to the heater 70 to heat andmelt the copper or copper alloy 22 in the upper chamber 74a (step S403).Also in this embodiment, the heating fluid may be allowed to flowthrough the passage 80 in the lower punch 64 together with the electricpower application to the heater 70 so that the inside of the refractoryvessel 66 is heated.

At a stage at which the melted matter (molten copper) of the copper orcopper alloy 22 in the upper chamber 74a arrives at a predeterminedtemperature, the upper punch 68 is moved downwardly to pressurize theinside of the upper chamber 74a up to a predetermined pressure (stepS404).

The melted matter (molten copper) of the copper or copper alloy 22 inthe upper chamber 74a, which has arrived at the predetermined pressure,is extruded by the pressure in the upper chamber 74a through the filtermember 110 toward the lower chamber 74b, and it is introduced into thelower chamber 74b, simultaneously with which SiC 20 installed in thelower chamber 74b is impregnated therewith.

At a stage of arrival at the end point previously set by timemanagement, in turn, the cooling fluid is allowed to flow through thepassage 80 in the lower punch 64 to cool the refractory vessel 66 fromlower portions to upper portions (step S405). Thus, the molten copperimpregnated into SiC 20 is solidified.

At the point of time of completion of the solidification, SiCimpregnated with the copper or copper alloy is extracted from therefractory vessel (step S406).

Also in the production method concerning this modified embodiment, it ispossible to efficiently impregnate SiC 20 with the copper or copperalloy 22, in the same manner as the production method according to thesecond embodiment. Also in this embodiment, after both of the moltencopper and SiC 20 are allowed to be in the pressure-applied state, theyare allowed to make contact with each other to perform the impregnatingtreatment. Accordingly, the pressure drop, which would occur upon thecontact of the both, can be minimized. Thus, it is possible to wellmaintain the pressure-applied state during the impregnating treatment.In the embodiment described above, the impregnating treatment isperformed at the negative pressure. However, the impregnating treatmentmay be performed at an ordinary pressure.

Next, a production method according to a third embodiment will beexplained with reference to FIGS. 16 and 17. Components or partscorresponding to those shown in FIG. 10 are designated by the samereference numerals, duplicate explanation of which will be omitted.

The production method according to the third embodiment is substantiallythe same in principle as the production method according to the secondembodiment. However, the former is different from the latter in that inthe impregnating step, SiC 20 is allowed to make contact with the copperor copper alloy 22 at a negative pressure or at an ordinary pressure,and they are subjected to a heating treatment to melt the copper orcopper alloy 22.

Specifically, as shown in FIG. 16, the production method according tothe third embodiment is different from the production method accordingto the second embodiment in that the filter 54 is not employed in therefractory vessel 66 of the hot press furnace 60 used in the productionmethod according to the second embodiment, but SiC 20 and the copper orcopper alloy 22 are introduced in this order from the bottom.

The production method according to the third embodiment is carried outby executing the steps shown in FIG. 17.

At first, SiC 20 and ingot of copper or copper alloy 22 are introducedinto the hollow section 74 of the refractory vessel 66 in this orderfrom the bottom (step S501).

Next, the hot press furnace 60 is tightly sealed. After that, the insideof the refractory vessel 66 is subjected to vacuum suction through thesuction pipe 72 so that the inside of the refractory vessel 66 is in anegative pressure state (step S502).

After that, an electric power is applied to the heater 70 to heat andmelt the copper or copper alloy 22 in the refractory vessel 66 (stepS503). During this process, the heating fluid may be allowed to flowthrough the passage 80 in the lower punch 64 together with the electricpower application to the heater 70 so that the inside of the refractoryvessel 66 is heated.

At a stage at which the melted matter (molten copper) of the copper orcopper alloy 22 in the refractory vessel 66 arrives at a predeterminedtemperature, the upper punch 68 is moved downwardly to pressurize theinside of the refractory vessel 66 up to a predetermined pressure (stepS504).

The melted matter (molten copper) of the copper or copper alloy 22,which has arrived at the predetermined pressure, is impregnated into SiC20 in accordance with the pressure in the refractory vessel 66.

At a stage of arrival at the end point (point of time at which theimpregnation of the molten copper into SiC 20 is in a saturated state)previously set by time management, in turn, the cooling fluid is allowedto flow through the passage 80 in the lower punch 64 to cool therefractory vessel 66 from lower portions to upper portions (step S505).Thus, the molten copper impregnated into SiC 20 is solidified. Thepressure-applied state in the refractory vessel 66, which is effected bythe upper punch 68 and the lower punch 64, is maintained untilcompletion of the solidification.

At the point of time of completion of the solidification, SiC 20impregnated with the copper or copper alloy 22 is extracted from therefractory vessel 66 (step S506).

Also in this production method, SiC 20 and the copper or copper alloy 22are heated while performing sufficient deaeration to melt the copper orcopper alloy 22 in the state of contact between the copper or copperalloy 22 and SiC 20, followed by giving the pressure-applied state inthe refractory vessel 66. Further, the pressure-applied state ismaintained until completion of the cooling operation. Thus, it ispossible to efficiently impregnate SiC 20 with the copper or copperalloy 22.

In the embodiment described above, the metal with which SiC 20 isimpregnated is the copper or copper alloy 22. However, the copper maycontain impurities such as 0.001% by weight to 0.1% by weight of Ca, Ag,Cd, Zn, Au, Pd, In, Ga, Pt, Cr. Ge, Rh, Sb, Ir, Co, As, Zr, Fe, Sn, Mn,P, and Pb, and gas components. Of course, the copper may be pure copper.

SiC 20 is used as the porous sintered compact to be subjected to theimpregnation. However, those usable is not limited to SiC but includeporous sintered compacts having the bending strength of not less than 10MPa, such as AlN, Si₃ N₄, B₄ C, and BeO.

It is a matter of course that the composite material for heat sinks forsemiconductor devices and the method for producing the same according tothe present invention are not limited to the embodiments describedabove, and the present invention may be embodied in other various formswithout deviating from the gist or essential characteristics thereof.

What is claimed is:
 1. A composite material for heat sinks forsemiconductor devices, comprising:a porous sintered ceramic compacthaving an average open pore diameter of not greater than 50 μm, saidporous sintered ceramic compact being impregnated with a metal composedof copper or a copper alloy, said porous sintered ceramic compact beingobtained by pre-calcinating a porous body having a coefficient ofthermal expansion which is lower than a coefficient of thermal expansionof copper so that a network structure is formed; wherein a ratio of saidcopper or said copper alloy to said porous sintered ceramic compact is20% by volume to 70% by volume, and said composite material has acoefficient of thermal expansion at 200° C. that is lower than atheoretical coefficient of thermal expansion which is stoichiometricallyobtained on the basis of a ratio between said copper or said copperalloy and said porous sintered ceramic compact.
 2. The compositematerial for heat sinks for semiconductor devices according to claim 1,further comprising a reaction layer, at an interface between said poroussintered ceramic compact and said metal, having a thickness not morethan 5 μm.
 3. The composite material for heat sinks for semiconductordevices according to claim 2, wherein the thickness of said reactionlayer is not more than 1 μm.
 4. The composite material for heat sinksfor semiconductor devices according to claim 1, wherein said poroussintered ceramic compact comprises at least one or more compoundsselected from the group consisting of SiC, AlN, Si₃ N₄, B₄ C, and BeO.5. The composite material for heat sinks for semiconductor devicesaccording to claim 1, wherein said composite material has an averagecoefficient of thermal expansion in a range from room temperature to200° C. of 4.0×10⁻⁶ /° C. to 9.0×10⁻⁶ /° C., and a coefficient ofthermal conductivity at room temperature of not less than 180 W/mK. 6.The composite material for heat sinks for semiconductor devicesaccording to claim 1, wherein said average open pore diameter of saidporous sintered ceramic compact is 0.5 to 50 μm.
 7. The compositematerial for heat sinks for semiconductor devices according to claim 6,wherein said average open pore diameter of said porous sintered ceramiccompact is distributed by not less than 90% in a range of 0.5 to 50 μm.8. The composite material for heat sinks for semiconductor devicesaccording to claim 1, wherein said composite material has a bendingstrength of not less than 10 MPa.
 9. The composite material for heatsinks for semiconductor devices according to claim 1, wherein saidcopper or said copper alloy contains one or more elements selected fromthe group consisting of Be, Al, Si, Mg, Ti, and Ni in a range up to 1%,inevitable impurity components and gas components.